Top Quark Pair plus Large Missing Energy at the LHC

TL;DR

This paper investigates methods to detect new physics involving top-quark pairs with large missing energy at the LHC, focusing on distinguishing signals from backgrounds and identifying the nature of top partners.

Contribution

It introduces a strategy to reconstruct signals of top partners decaying to top quarks plus missing energy, and proposes techniques to differentiate scalar from fermionic top partners.

Findings

Reconstructed a transverse mass to separate signal from background.

Defined a top quark mass variable indicating new physics.

Mapped the LHC discovery reach for different mass parameters.

Abstract

We study methods of extracting new physics signals in final states with a top-quark pair plus large missing energy at the LHC. We consider two typical examples of such new physics: pair production of a fermionic top partner (a $T'$ in Little Higgs models for example) and of a scalar top partner (a $\tilde{t}$ in SUSY). With a commonly-adopted discrete symmetry under which non Standard Model particles are odd, the top partner is assumed to decay predominantly to a top quark plus a massive neutral stable particle $A^0$. We focus on the case in which one of the top quarks decays leptonically and the other decays hadronically, $pp \to {\tt} A^0A^0 X \to bj_1j_2 \bar b\ell^- \bar\nu A^0A^0 X + c.c.$, where the $A^0$s escape detection. We identify a key parameter for the signal observation: the mass splitting between the top partner and the missing particle. We reconstruct a transverse mass for the lepton-missing transverse energy system to separate the real $W$ background from the signal and propose a definition for the reconstructed top quark mass that allows it to take unphysical values as an indication of new physics. We perform a scan over the two masses to map out the discovery reach at the LHC in this channel. We also comment on the possibility of distinguishing between scalar and fermionic top partners using collider signatures.